Such a system is known from U.S. Pat. No. 4,863,374. In the semiconductor industry, and also in other processes in which a very small quantity of gas or other medium is metered into a reactor in order to allow the process in question to proceed, it is frequently important for the reduced pressure on the discharge side to be kept accurately constant. If such a reduced pressure varies excessively, the result is that the flow rate and residence time of the medium in question through the reactor varies, and consequently the end product is adversely affected. This applies in particular if the discharge system for a reactor of this kind is combined with the discharge system of other reactors. Changes in the process conditions in those other reactors could have an adverse effect on the process conditions in the first reactor. This applies in particular if the discharge system is provided with a central suction device, such as a fan, and in the event of the flow rate through one reactor increasing or falling, the flow rate in another reactor will be reduced or increased, respectively.
A number of proposals have been made in the prior art in order to avoid this problem.
A first proposal comprises arranging a butterfly valve in the connection between the outlet from the reactor and the discharge line. This butterfly valve is set manually to a fixed value.
According to another proposal, a butterfly valve of this kind was likewise used in the connection between discharge line and outlet, but this valve can be controlled during the process. To do this, it is necessary to use a complicated pressure gauge and to convert the signal which it measures into a movement of the valve, using electric, hydraulic or pneumatic means. A solution of this kind is particularly expensive if there are a number of discharge lines and restrictor devices of this kind present. Moreover, there are considerable adjustment problems and it is not readily possible to fix the pressure to an absolute value.
Moreover, it is necessary when using certain toxic gases to make the latter less hazardous before they are released into the free atmosphere. This can be achieved by the provision of expensive absorbing agents. Another proposal which is known in the prior art is the admixture of air or other gases from the atmosphere, with the result that the concentration of hazardous products of this kind is reduced. To this end, a separate air-metering device is proposed, the supply of air preferably taking place downstream of the suction device, in order not to interfere unnecessarily with operation of the discharge system.
It is proposed in U.S. Pat. No. 4,863,374 to arrange an electrically operated valve in the discharge line to the discharge duct. This valve functions to suck in cold air, in order for this to be admixed to the gases emanating from a furnace. In order to protect the discharge system against excessively high temperatures, the supply of cold ambient air to the outlet is controlled as a function of the temperature in the furnace. There is no mention of controlling the pressure prevailing in the furnace, and in view of the aim of the furnace, namely to heat ceramic objects, this also does not appear to be important.